Method of improving the control behavior of a braking system

ABSTRACT

A method of improving the control behavior of a controlled brake system, wherein the rotational behavior of the individual vehicle wheels is measured and evaluated for determining control quantities, and wherein slip threshold values for the commencement of the control are predetermined, includes that the slip threshold of a wheel is increased for a predefined time interval as a function of the acceleration after preceding instability in order to suppress the effects of road surface irregularities. This increase may be carried out in two steps as a function of the exceeded acceleration limit values.

BACKGROUND OF THE INVENTION

The present invention relates to a method of improving the controlbehavior of an anti-lock and/or traction-slip controlled brake system,wherein the rotational behavior of the individual wheels is measured andevaluated for determining a vehicle reference speed, the wheel slip, thedeceleration and acceleration of the individual wheels and, if required,other control quantities, and wherein threshold values of the wheel slipare predefined for the commencement of the control.

Brake systems with electronic anti-lock control (ABS) are nowadaysincluded in the standard equipment of a great number of automotivevehicles. Also, extension of the ABS to traction slip control is no rareoccurrence. In systems of this type the rotational behavior of theindividual wheels is measured by means of wheel sensors and evaluated byelectronic circuits to generate the control quantities and brakingpressure control signals. Among these control quantities which resultfrom the wheel rotational behavior are mainly wheel slip, wheel speed,wheel deceleration and wheel acceleration and a so-called vehiclereference speed which is defined by logical combining of the individualwheel speeds.

Identifying the actual control situation from the data provided by theindividual wheel sensors and the consistent controlling of the brakingpressure for anti-lock control or traction slip control is alwaysdifficult when the interpretation of the rotational behavior of thewheels does not permit a definite indication of the instantaneous roadsituation and the vehicle behavior. Further, it is known thatdisturbances on the road surface or road surface irregularities of anytype may cause misinformation of the controller or the evaluatingcircuit and undesirable control operations or variations in the controlwhich have adverse effects on the control. This is because decelerationand acceleration actions or slip signals occur on the individual vehiclewheels due to the road disturbances which are interpreted by thecontroller as instabilities of the wheel run.

Road surface disturbances or irregularities of the abovementioned typeimpair the ABS function. The ABS may even respond when the brake is notapplied as soon as the ABS identification thresholds, which are slip ordeceleration thresholds, are exceeded. When the brake is appliedsubsequently in the current ABS control, relatively great yaw torquesmay be caused which impair the driving stability of the vehicle. Also,the vehicle deceleration which is achievable in the given situation inan optimal braking operation will not be reached. Similar difficultiesor discrepancies from the optimal behavior occur during traction slipcontrol operations.

Therefore, an object of the present invention is to suppress the effectsof various types of road irregularities on the control function of anABS or TCS system and to thereby improve the control behavior of the ABSor TCS system. The objective is to achieve this improvement withoutdisadvantages for the control sensitivity and control quality in othersituations.

SUMMARY OF THE INVENTION

It has been found that this object is achieved by a method includingthat the slip threshold of a wheel is increased for a predetermined timeinterval as a function of the acceleration of the respective wheel afterpreceding instability. Appropriately, the slip threshold is increasedproportionally to the acceleration of the wheel. The slip threshold riseis effected when the wheel acceleration exceeds a predetermined limitvalue. The method of the present invention is based on the idea that thewheel acceleration values after a preceding instability are not only dueto the reduction of the brake torque. Rather, the values also depend onwheel load variations which influence the pressure modulation. There iseven the imminent risk of the wheels losing their road contact underextreme conditions.

Under these conditions, deceleration (if one reacted in a conventionalmanner, i.e., by a corresponding pressure reduction) would depend on thearrangement of the disturbance values, or the irregularities on the roadsurface, and other coincidences. Therefore, according to the presentinvention, the sensitivity of the control is temporarily reduced byraising the threshold. The disturbances caused by road irregularitiesare thereby suppressed actively by an increased rate of brake torque.This is also advantageous in terms of achieving a short stoppingdistance.

In another aspect of the present invention, several acceleration limitvalues of a different magnitude are predefined, and the slip thresholdsare stepwise increased as a function of the exceeding of theacceleration limit values. It has proved to be especially appropriate topredefine two acceleration limit values and to increase the slipthresholds by a predetermined, relatively low value when a first, lowacceleration limit value is exceeded, and by a predetermined,comparatively high value when a second, higher acceleration limit valueis exceeded.

The predetermined time intervals for the increase of the slip thresholdsare in the order between 50 and 200 ms.

Further, according to the method of the present invention, the slipthresholds are reset after their increase or rise due to the exceedingof the predetermined acceleration limit values as soon as theacceleration of the wheel falls below a predetermined resettingthreshold, or does not reach the acceleration threshold any more, uponexpiry of the predetermined time interval. The level of the resettingthreshold depends on the step or the value to which the slip thresholdwas raised. When the slip threshold, due to relatively significantdisturbances or road irregularities, was raised to a comparatively highvalue, a relatively high resetting threshold applies. A low value forthe resetting threshold rules when the road irregularities had causedonly a relatively insignificant rise of the slip threshold.

Further features, advantages and possible applications of the presentinvention can be seen in the following description of further details,making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a schematically simplified block diagram view of a circuit forimplementing the method of the present invention on an ABS;

FIG. 2 is a diagram showing the speed of a wheel (FIG. 2A), the timevariation of the increase (FIG. 2B) and the stepwise increase of theslip threshold (FIG. 2C) when the method of the present invention isemployed.

DETAILED DESCRIPTION OF THE DRAWINGS

The block circuit of FIG. 1 represents the main components of theelectronic circuit of an electronically controlled ABS. The inputsignals of the control system are produced by wheel sensors S1 to S4from which signals or data are derived in a conditioning circuit 1 thatrepresent the speed variation v₁ to v₄ of the individual vehicle wheels.Mainly the first and second time derivative v₁ to v₄ and v₁ to v₄ isproduced in an evaluating circuit 2, and the slip λ₁ to λ₄ of theindividual wheels is calculated. As a reference quantity for this slipcalculation a vehicle reference speed v_(REF) correspondingapproximately to the vehicle speed is required. The vehicle referencespeed is determined in a circuit 3 by logically combining the individualspeed signals v₁ to v₄.

In a circuit 4 which is represented as an ABS-logic circuit, brakingpressure control signals are produced by data processing on the basis ofcomplicated algorithms and by evaluating all data available from thevalues measured on each individual wheel and the derived values. Thesignals are sent by way of a valve actuation control 5 to a circuitblock 6 which includes braking pressure actuators, for example,electromagnetically controllable hydraulic valves. It is, for example,known to assign an electrically or electromagnetically controllableinlet and outlet valve to each controlled vehicle wheel for brakingpressure control and modulation.

Appropriately, ABS circuit systems of the type illustrated in FIG. 1 arerealized by way of one or more microcomputers, microcontrollers, orsimilar means.

The additional components for implementing the method of the presentinvention are represented in FIG. 1 by an additional circuit 7 which is,of course, realized in a data processing system by corresponding programsteps. In this circuit 7, the time derivatives v₁ to v₄, especially thereacceleration signals, of the individual vehicle wheels are analyzed.The index `i` in FIG. 1 shows that the time derivative or the wheelacceleration V_(i) for each wheel i (i=1 . . . 4) is produced andevaluated.

As will be described hereinbelow, the existence of disturbances,especially road irregularities, is concluded by way of circuit 7 fromthe magnitude of the acceleration or reacceleration of a wheel afterpreceding instability. To suppress the effects of such roadirregularities on the control, the thresholds of commencement of theanti-lock control are varied as described by way of the diagrams shownin the FIG. 2 embodiment.

Another circuit 8 (shown in dotted lines) indicates the possibility ofemploying still further measures (realized by hardware or software) toimprove the control, for example, when so-called bumps, road surfacedepressions or washboard roads occur, in a fashion which is known or hasalready been described in former applications.

The method of the present invention is basically appropriate for controlsystems with variable control commencement thresholds, especiallyvariable slip thresholds. FIG. 2 relates to an example of a system ofthis type. FIGS. 2A to 2C are used to illustrate the operation of themethod of the present invention. The diagram `A` on the top illustratesthe speed variation v_(R) of a vehicle wheel during a control operation,wherein each phase of instability is followed by a relatively greatreacceleration of the wheel. This wheel speed variation might be causedby road surface irregularities. The reacceleration of the wheel isanalyzed according to the present invention. In the first case, at timet approximately, a comparatively great reacceleration V_(R) =a₁ isdetermined which exceeds a predetermined, relatively low accelerationlimit value a_(g1). At time t₁, therefore, a slip threshold which rulesthe commencement of the control is increased by a defined amount whichis referred to as slip threshold `offset`, i.e., to the first stepaccording to FIG. 2C. Unless other events occur, this increase appliesfor a predetermined time interval T, which is represented by the countercontent Z (shown in FIG. 2B) which is reset after the start at time t₁linearly within the time interval T. The acceleration value at time t₁is shown in FIG. 2A by a tangent to the wheel speed curve v_(R).

After the repeated instability and reacceleration of the wheel, thespeed v_(R) of which is shown, a reacceleration v_(R) =a₂ is determinedat time t₂, still prior to the expiry of the predetermined time intervalT. The reacceleration exceeds the reacceleration at time t₁ and asecond, higher acceleration limit value a_(g2). The result is anincrease of the slip threshold related to this wheel by a still higheramount, i.e. a rise to the second step according to FIG. 2C. Further,the counter Z (FIG. 2B) is raised to the starting value Z₀ at time t₂ sothat the counter which defines the time interval T is started again.

After a repeated wheel deceleration and reacceleration of the vehiclewheel R under review, a wheel acceleration V_(R) =a₃ is determined attime t₃ which is less than the acceleration at time t₂, however, isstill in excess of a predetermined resetting limit value or in excess ofa resetting threshold a_(RS). Consequently, the rise of the slipthreshold to the second step (see FIG. 2C) is maintained, and thecounter (FIG. 2B) is started again.

At time t₄, the resetting threshold a_(RS) falls below, whereupon ontermination of the predetermined time interval T, related to thestarting point t₃, the increase of the slip threshold is terminated attime t₅. Thus, the original sensitivity or the original slip thresholdof the control system will predominate again.

Two different values apply for the resetting threshold a_(RS) (whichcannot be taken from the embodiment shown in FIGS. 2A to C, however). Atop threshold value a_(RS2) determines the removal of the slip thresholdincrease, provided that the second step of the slip threshold rise hadbeen achieved before (see FIG. 2C). A second, lower resetting thresholda_(RS1) which is below the lower acceleration limit value a_(g1) comesinto effect if previously the slip threshold had been increased to thefirst step and, subsequently, the reacceleration did not reach the slipthreshold.

In a special embodiment, the following values were fixed for theacceleration limit values, the resetting thresholds and for thepredetermined time interval T:

    a.sub.g1 ˜7 g (6.5 . . . 7.7 g)

    a.sub.g2 ˜9 g (8.5 . . . 9.5 g)

    a.sub.RS1 ˜4 g (3.5 . . . 4.5 g)

    a.sub.RS2 6 g (5.5 . . . 6.5 g)

    T˜110 msec (5.0 . . . 200 msec)

`g` refers to the constant of acceleration due to gravity (g=9.81 m/s²).

It is, of course, also possible to increase the slip threshold in morethan two steps in dependence on the reacceleration of the respectivevehicle wheel. On the other hand, it is deemed sufficient in many casesto provide one single step for the threshold rise.

Further, it may be expedient to provide different threshold offsets orthreshold rise steps for the driven wheels and the non-driven wheels.This prevents that vibrations of the transmission line cause an unwantedthreshold rise and, thus, a relatively insensitive control. The specialincrease values depend on the respective vehicle construction.

What is claimed is:
 1. A method of improving the control behavior of anelectronically controlled brake system, wherein the rotational behaviorof individual vehicle wheels is measured and evaluated for determining avehicle reference speed, wheel slip, wheel deceleration andacceleration, and, if required, other control quantities,wheel slipthreshold values being predefined for the commencement of a control, thewheel slip threshold being increased for a predetermined time intervalas a function of the wheel acceleration after preceding instability ofthe respective wheel, the wheel slip threshold rise being effected whenpredetermined limit values of wheel acceleration are exceeded whereinseveral acceleration limit values of a different magnitude arepredefined, and wherein the wheel slip threshold is stepwise increasedas a function of the excess of the acceleration limit values.
 2. Amethod as claimed in claim 1,wherein two acceleration limit values arepredetermined, and wherein the slip threshold is increased by apredetermined, relatively low value in a first step when a first, lowacceleration limit value is exceeded, and by a predetermined,comparatively higher value in a second step when a second, higheracceleration limit value is exceeded.
 3. A method as claimed in claim1,wherein the predetermined time interval for the increase of the slipthreshold is in the order between 50 and 200 msec.
 4. A method ofimproving the control behavior of an electronically controlled brakesystem, wherein the rotational behavior of individual vehicle wheels ismeasured and evaluated for determining a vehicle reference speed, wheelslip, wheel deceleration and acceleration, and, if required, othercontrol quantities,wheel slip threshold values being predefined for thecommencement of a control, the wheel slip threshold being increased fora predetermined time interval as a function of the wheel accelerationafter preceding instability of the respective wheel, the wheel slipthreshold rise being effected when predetermined limit values of wheelacceleration are exceeded, wherein the slip threshold, which wasincreased due to an excess of the predetermined acceleration limitvalue, is reset as soon as the acceleration of the wheel falls below apredetermined resetting threshold upon expiry of the predetermined timeinterval.
 5. A method as claimed in claim 4,wherein the level of theresetting threshold depends on the value to which the slip threshold wasraised.
 6. A method as claimed in claim 4, wherein the predeterminedtime interval for the increase of the slip threshold is in the orderbetween 50 and 200 ms.